1. The Field of the Invention
The present invention generally relates to a safety device used particularly in motor vehicles. More particularly this invention relates to a compressed gas inflator used to inflate an airbag or cushion during a collision.
2. The Relevant Technology
As individuals travel within a vehicle it is necessary to restrain them in case of a collision. Traditionally, a safety belt would secure an individual in place. The safety belt would laterally and diagonally cross a passenger or driver's body, and be anchored to the body of the car. Unfortunately, safety belts only give limited protection from head or neck injury during a collision and associated rapid deceleration.
With the advances in technology, it has become well known to protect a vehicle's occupant using an inflatable device, such as an airbag or cushion. An airbag is manufactured from a fabric bag which is rapidly inflated or expanded with gases which exit an inflatable device. These gases are termed exit gases. The airbag is activated when the vehicle encounters sudden deceleration, such as in a collision with another vehicle or structure. The airbag allows a reasonable deceleration of an occupant's body in a collision and prevents the impact of the head into the steering wheel or passenger side dashboard.
Prior to its inflation, the airbag is in a deflated and/or folded condition to minimize the space required to house the airbag. In the event of a collision, however, the airbag inflates in a matter of no more than a few milliseconds to provide occupant protection. The exit gases are supplied to the airbag by a device commonly referred to as "an inflator." An inflator may be of various sizes and/or configurations. For example, a stored gas inflator typically requires the storage of a highly pressurized gas. As a result of the high storage pressures, the walls of the gas storage chamber are relatively thick, thereby resulting in heavy and bulky inflator designs.
Another type of inflator is a pyrotechnic inflator. A pyrotechnic inflator utilizes a combustible gas generating material to create the exit gases used to inflate the airbag. The combustible gas generating material produces gases with high temperature, typically ranging from about 500.degree. F. (260.degree. C.) to 1200.degree. F. (649.degree. C.). These temperatures are dependent on numerous interrelated factors including the desired level of inflator performance, as well as the type and amount of gas generating material used. For example, the level of inflator performance is defined by numerous crash variables, and those variables define the size and configuration of the inflator and airbag. The variables may include the severity of the crash, the size and position of the occupants, and the temperature conditions that may affect the performance of the inflator.
Yet another type of inflator is a hybrid inflator. A hybrid inflator utilizes a combination of stored pressurized gas and a pyrotechnic gas generating material. The pyrotechnic gas generating material provides some exit gases to inflate the airbag. Furthermore, the pyrotechnic can be used to also heat and expand the stored gas to enhance its contribution to inflating the airbag. Some inflators will produce the required exit gases from a compressed liquid or gas, while others will generate the exit gases through a combination of material decomposition and gaseous reaction. Furthermore, hybrid inflators may also include a heating composition as part of the igniting assembly. The heating composition typically comprises of a non gas-producing material which heats the exit gases as they pass through the heating composition. The heating composition boosts the inflation rate of the exit gases and therefore increases the expansion of the gas flowing therethrough.
In addition to there being many types of inflators, there are also numerous types of igniting assemblies or squibs which produce the exit gases. Squibs may be, for example, a pyrotechnic, a bridgewire, a spark-discharge, a heated or exploding wire/foil, or a semi-conductor bridge (SCB). Furthermore, each squib may incorporate a projectile type device to start the inflation of the airbag.
Regardless of the type of inflator or squib that is used in the particular inflatable device, the manufacture of an inflator requires joining of numerous components together. For example, in a stored gas inflator, it is necessary to weld a burst disk between a gas containing tube and a diffuser. Furthermore, it is also necessary to weld an end closure/igniting assembly to the gas containing tube. The processes traditionally used to join the above components have, however, a number of limitations.
Traditionally, the inflator components are joined together through a multi-step process. The multi-step process may include, for example in a stored gas inflator, first, forming an inflator body, usually from a metal tube; second, welding an end closure/igniting assembly to the body; third, welding a burst disk to the body; fourth, welding a diffuser to the top of the tube, fifth, filling the inflator with a gas and sealing the fill port; and sixth, checking the inflator for leaks.
One of the most time consuming steps in manufacturing an inflator is the manufacture of the end closure/igniting assembly and connecting the assembly to the inflator body. For example, in a glass to metal igniting assembly, the end closure is formed from a glass body which is fixed within a metal housing. A number of initiator pins are located within the glass body which are configured to connect with an external electrical source. Once the glass body is fixed to the metal housing, a bridging element is welded, soldered, brazed or bonded to the initiator pins. After the bridging element is securely fixed in place a pyrotechnic mixture is added to the bridging element. In some circumstances an output charge is also added to the inflator. The bridging element acts as an initiator either to ignite a pyrotechnic or, in the case of a hybrid or stored gas inflator, to activate the pressurized gases within the inflator. Finally, the end closure is welded or crimped to the bottom of the body to form the completed inflator.
This process of manufacturing both the end closure/igniting assembly and the inflator is time consuming because of the precision required for pressure vessel construction. In addition, there are a number of problems which may arise during the manufacturing process, such as during set-up or manufacturing operations.
Since there are many different welding and bonding operations, there is an increased likelihood for defects in the completed inflator. Defects may occur in the body of the inflator, in the welded areas of the inflator, or within the igniting assembly or the diffusing assembly. In particular, microcracks may form in the bonds between the glass body and the end closure, in the welds between the end closure and the inflator body, between the inflator body and the diffuser, and/or around the fill port.
Microcracks cause catastrophic failure of pressure vessels, such as inflators. As an inflator is used, the pressurized gas within the inflator body transfers energy to any microcrack contained with in the inflator body or joints. In a collision, the igniting assembly activates the gases and the pressure within the inflator increases. The increased pressure provides additional energy to the microcracks thereby increasing crack growth. If the pressure within the inflator becomes so great that a critical stress is reached, the pressure vessel catastrophically fails. Therefore, the inflator does not inflate the airbag and protect the occupant.
It would therefore be an advance to provide an inflator which reduces the possibility of structural defects, such as microcracks and leaks, while still providing an economical and efficient process of manufacture.